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Previously I had thought of the custom of firstborn children in the context of ancient fertility rites. That is, a young girl was expected to achieve her first pregnancy our of wedlock and as part of an established religious festival. The resulting offspring was offered up to the gods to ensure fertility of the land and guard against overpopulation. If the family of the girl had the means, the child could perhaps be saved with an appropriate donation to the priests.

Within the royal family this practice partly compenstated for habitual incest, because the firstborn was usually redeemed rather than literally sacrificed to the gods. However, Biblical accounts indicate that the firstborn was also generally recognized as the legal heir of the girl's future husband. There is a dynastic element to this custom that I hadn't fully recognized before. Specifically, the Great King could use this custom to advantage by imposing his right as "Godfather" to produce the firstborn child of high-ranking women. When these women were married, the firstborn sons would be established as heirs and thereby the male lineage of the Great King was grafted onto that of rival royal males and other high-ranking men.

I think this is precisely the scenario of Alexander the Great. He would have been the firstborn son of his mother Olympia by the Great King ("Zeus"), namely Artaxerxes II. Artaxerxes II and Phillip II of Macedon are known to have entered into an alliance, and the price for Phillip was the acceptance of Olympia as his queen and Alexander as his heir. The alliance also included an expectation that Phillip would subdue the rest of Greece. However, the young Alexander was obviously more motivated than his legal father Phillip, as he stood to be the primary beneficiary of such a conquest.

We should also expect to find examples of primogeniture in the Roman era. As various imperiled rulers of the East appealed to Rome for aid against their rivals, the Romans typically agreed to intervene, but we now know that their conditions would have included the right to produce heirs. It would now not surprise me at all to find that the eldest son of Herod, namely Antipater, was actually the true son of a Roman aristocrat such as Appius, Caesar, Agrippa, or Marc Antony. The same expectation would apply to other wives of Herod such as Mariamne, Mariamne II/Cleopatra, Pallas, etc.